Merge pull request #724 from Unity-Technologies/Fix-issue-with-tessellation-and-reflection-probe

HDRenderPipeline: Fix reflection probe and tessellation (object was badly culled)

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Camera/HDCamera.cs

             }
 
             // Near, far.
-            frustumPlaneEquations[4] = new Vector4( camera.transform.forward.x,  camera.transform.forward.y,  camera.transform.forward.z, -Vector3.Dot(camera.transform.forward, relPos) - camera.nearClipPlane);
-            frustumPlaneEquations[5] = new Vector4(-camera.transform.forward.x, -camera.transform.forward.y, -camera.transform.forward.z,  Vector3.Dot(camera.transform.forward, relPos) + camera.farClipPlane);
+            // We need to switch forward direction based on handness (Reminder: Regular camera have a negative determinant in Unity and reflection probe follow DX convention and have a positive determinant)
+            Vector3 forward = viewParam.x < 0.0f ? camera.transform.forward : -camera.transform.forward;
+            frustumPlaneEquations[4] = new Vector4( forward.x,  forward.y,  forward.z, -Vector3.Dot(forward, relPos) - camera.nearClipPlane);
+            frustumPlaneEquations[5] = new Vector4(-forward.x, -forward.y, -forward.z,  Vector3.Dot(forward, relPos) + camera.farClipPlane);
 
             m_LastFrameActive = Time.frameCount;
         }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitDataMeshModification.hlsl

         input.uv3,
     #else
         float2(0.0, 0.0),
-    #endif            
+    #endif
-    #endif      
+    #endif
-    
+
 #ifdef _VERTEX_WIND
     float3 rootWP = mul(GetObjectToWorldMatrix(), float4(0, 0, 0, 1)).xyz;
     ApplyWindDisplacement(positionWS, normalWS, rootWP, _Stiffness, _Drag, _ShiverDrag, _ShiverDirectionality, _InitialBend, input.color.a, time);
     // For tessellation we want to process tessellation factor always from the point of view of the camera (to be consistent and avoid Z-fight).
     // For the culling part however we want to use the current view (shadow view).
     // Thus the following code play with both.
-    float frustumEps = -maxDisplacement;
-    
+    float frustumEps = -maxDisplacement; // "-" Expected parameter for CullTriangleEdgesFrustum
+
     // TODO: the only reason I test the near plane here is that I am not sure that the product of other tessellation factors
     // (such as screen-space/distance-based) results in the tessellation factor of 1 for the geometry behind the near plane.
     // If that is the case (and, IMHO, it should be), we shouldn't have to test the near plane here.
     if (_TessellationBackFaceCullEpsilon > -1.0) // Is back-face culling enabled ?
     {
         // Handle transform mirroring (like negative scaling)
+        // Note: We don't need to handle handness of view matrix here as the backface is perform in worldspace
         float winding = unity_WorldTransformParams.w;
         faceCull = CullTriangleBackFace(p0, p1, p2, _TessellationBackFaceCullEpsilon, GetCurrentViewPosition(), winding); // Use shadow view
     }
         input.texCoord3,
     #else
         float2(0.0, 0.0),
-    #endif            
+    #endif
-    #endif      
+    #endif
         );
 #endif // _TESSELLATION_DISPLACEMENT
 }